Abstract
To determine the diffusion and sorption properties of radionuclides in intact crystalline rocks, a new electromigration device was built and tested by running with I− and Se(IV) ions. By introducing a potentiostat to impose a constant voltage over the studied rock sample, the electromigration device can give more stable and accurate experimental results than those from the traditional electromigration devices. In addition, the variation in the pH of the background electrolytes was minimised by adding a small amount of NaHCO3 as buffers. To interpret the experimental results with more confidence, an advection-dispersion model was also developed in this study, which accounts for the most important mechanisms governing ionic transport in the electromigration experiments. Data analysis of the breakthrough curves by the advection-dispersion model, instead of the traditional ideal plug-flow model, suggest that the effective diffusivities of I− and Se(IV) are (1.15 ± 0.06) × 10−13 m2/s and (3.50 ± 0.86) × 10−14 m2/s, respectively. The results also show that I− is more mobile than Se(IV) ions when migrating through the same intact rock sample and that their sorption properties are almost identical.
Highlights
The final disposal of spent nuclear fuel is considered to take place in deep geological repositories in many countries (De Cannière et al, 2010; Ewing, 2015; Posiva, 2013)
Many studies have been carried out to determine the sorption and diffusion properties of dominant radionuclides existing in nuclear waste, such as Cs+, Sr2+, I− and SeO32−, by batch sorption experiments (Li et al, 2018; Söderlund et al, 2016a, 2016b), block scale diffusion experiments (García-Gutiérrez et al, 2006; Ikonen et al, 2016b), through-diffusion experiments (Puukko, 2014; Tachi et al, 2015), electromigration experiments (André, 2009; André et al, 2009a; Löfgren and Neretnieks, 2006; Puukko et al, 2018) and He gas through diffusion experiments (Kuva et al, 2015, 2016)
The sample is fine to medium grained granitic rock without cm-scale micro fractures in it The porosity including flow, diffusion and residual parts is defined by Norton and Knapp (1977)
Summary
The final disposal of spent nuclear fuel is considered to take place in deep geological repositories in many countries (De Cannière et al, 2010; Ewing, 2015; Posiva, 2013). The sorption on the pore surfaces of the bedrock and the diffusion into the low-porous rock matrix are the two most significant processes that retard radionuclides migrating through the water conducting fractures of the crystalline rock (Dai et al, 2007; Grisak and Pickens, 1980; Neretnieks, 1980; Posiva, 2013; Séby et al, 1998) For this reason, many studies have been carried out to determine the sorption and diffusion properties of dominant radionuclides existing in nuclear waste, such as Cs+, Sr2+, I− and SeO32−, by batch sorption experiments (Li et al, 2018; Söderlund et al, 2016a, 2016b), block scale diffusion experiments (García-Gutiérrez et al, 2006; Ikonen et al, 2016b), through-diffusion experiments (Puukko, 2014; Tachi et al, 2015), electromigration experiments (André, 2009; André et al, 2009a; Löfgren and Neretnieks, 2006; Puukko et al, 2018) and He gas through diffusion experiments (Kuva et al, 2015, 2016). For the electromigration experiments under consideration, the full solution of the advection-dispersion model can be written in the Laplace domain as:
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